Digital television tape recording

ABSTRACT

The present invention relates to digital television tape recording and more particularly to such recording using helical scan apparatus of the segmented type i.e. where one field of video is recorded by a plurality of rotations of the headwheel. 
     The invention also provides apparatus for recording or playing back a digital television signal comprising a helical scanning device including a drum which may be a rotatable headwheel provided with a plurality of recording/playback heads, guide means for wrapping a magnetic recording tape around a portion of the circumference of the drum, means for rotating the headwheel, and means for moving the tape around the surface of the drum whereby one field of a television video picture is recorded or played back by a plurality of rotations of the headwheel, characterized in that the recording/playback heads are disposed on the headwheel.

This application is a continuation of application Ser. No. 438,878,filed as PCT GB82/00030 Feb. 5, 1982, published as WO82/02810, Aug. 19,1982, §102(e) date filed Oct. 5, 1982.

The present invention relates to digital television tape recording, andmore particularly to such recording using helical scan apparatus of thesegmented type, i.e., where one field of video is recorded by aplurality of rotations of the headwheel.

In this specification the expression "plurality of rotations" means morethan one rotation and includes fractional rotations in excess of one,e.g., one and one half rotations.

Digital television tape recording has already been the subject ofconsiderable attention, but most work has been concentrated on theproblems of high bit rates and packing densities and on error correctionand concealment. While many of the problems have been largely solved,there is still a need for a format for digital television recordingwhich will provide for improved error concealment or correction.Further, the requirements for recording the sound channel associatedwith the video has been largely ignored.

The present invention provides apparatus for recording or playing back adigital television signal comprising a helical scanning device includinga drum which may be rotatable, a rotatable headwheel provided with aplurality of recording/playback heads, guide means for wrapping amagnetic recording tape around a portion of the circumference of thedrum, means for rotating the headwheel, and means for moving the tapearound the surface of the drum whereby one field of a television videopicture is recorded or played back by a plurality of rotations of theheadwheel characterised in that the recording/playback heads aredisposed on the headwheel and the degree of wrap of the tape around thedrum is such that for one complete revolution of the headwheel there isa period of time during which there is no head in operative contact withthe tape, and storage means are provided for receiving information inthe form of digital words to be recorded or played back for altering thetiming of the information, whereby on recording all the informationreceived is recorded on playback the information on the tape isrecovered and returned to the timing relationship existing for theoriginal information.

An advantage of this apparatus is that it records the video informationon the tape in such a way as to leave gaps into which the audioinformation can be inserted. Further, at least one of the video headscan also be used for audio.

In order that the present invention be more readily understood,embodiments thereof will now be described by way of example withreference to the accompanying drawings, in which:

FIG. 1 shows diagrammatically a head arrangement and degree of tape wrapaccording to the present invention;

FIG. 2 shows a diagram representing how digital information relating toone line of video is recorded and played back according to the presentinvention;

FIG. 3 shows a diagram similar to FIG. 2 for another proposal accordingto the invention;

FIG. 4 shows a diagram similar to FIG. 2 but forthe audio signalassociated with the line of video;

FIG. 5 shows a block diagram of apparatus for recording a video signalaccording to the present invention.

In the description which follows, it is assumed that segmental helicalscan recording is well known and it will therefore not be described indetail. Further, the description is given in relation to recording onefield of a television signal, it being understood that a televisionframe is made up to two interlaced fields.

Referring now to FIG. 1, for the purpose of this consideration it hasbeen assumed that the source data rate for video will be 216 Mbit/s.This corresponds to sampling the luminance at 13.5 MHz and each colourdifference channel at 6.75 MHz. These particular frequencies areintegral multiples of both 525 and 625 horizontal frequencies.

Further assumptions are that a 200:1 ratio between head and tape speedwill be used under normal conditions, for example 1600 and 8 in/srespectively, and also that picture in shuttle is required at 30 timesnormal speed. The data rate recovered from tape under these conditionswill be as if the 216 Mbit/s source data rate had been increased to 247.This is sufficiently high that at least three heads are required inoperative contact with the tape.

FIG. 1 shows a cluster of four heads and a tape wrap angle of 270°. Thuswith 4 heads in contact with the tape for 3/4 of the time, this isequivalent to 3 heads in contact with the tape at all times which wouldbe the more usual approach. The use of clustered heads means that bufferstorage is needed due to there being no head in contact with the tapefor 1/4 of the time. The buffer storage is used to compress the digitalinformation to be recorded into groups and afterwards stretch it outagain. This will introduce delay in the video signal and this delay canbe used to advantage for recording the audio signal as will be describedlater.

The number of heads in a cluster is dependent on the maximum data rateper track which can be tolerated having regard to tape consumption.Thus, in some circumstances only 3 heads may be needed as will bedescribed later in relation to FIG. 3.

The angle of 270° that is shown for the clustered head approach givesthe equivalent of three heads in continuous contact but the 270° can bechanged. For example 240° gives the equivalent of 22/3 heads whilst 300°would give the equivalent of 31/3 heads in continuous contact. Suchvariations may be beneficial to optimise the format.

Two important advantages of the clustered head approach is that thefailure of a channel perhaps due to a head clog, would produce a smallerlevel of impairment and that this impairment would be uniformly spreadover the picture. This should be contrasted to a larger impairment whichwill appear only on bands of the picture for any equispaced approach;the larger impairment and in particular the banding would be much moreapparent.

Assuming that four heads are used the average source bit-rate per trackwould be 54 Mbit/s. If a 270° tape wrap is used for video this has to beincreased by a third giving 72 Mbit/s source bit-rate per track whilstthe head is actually recording video.

This would require a head-to-tape speed equal to twice that of the SMPTE`C` format. Obviously it would be impractical to double the drumdiameter. It would be better to have two revolutions per field with thesame drum diameter as used for the `C` format but probably the bestsolution would be three revolutions per field and to decrease the drumdiameter to about 2/3 of the `C` format. This gives a higher pressurebetween tape and head and, allowing for 270° wrap, roughly halves thetrack length. Having three rotations per field does mean that trackfollowing servo has to have a greater range to yield satisfactory slowmotion but this should not cause serious problems. However, themagnitude of this problem would increase with the number of rotationsper field if the wrap angle is kept fixed.

Making the assumption of three rotations per field and that 270°rotation will be used for video, how should the buffer storage bearranged so that all the video information is bunched into the 270°bursts.

Because three rotations per field are assumed, it is necessary to divideup each field of video into three groups of lines. Also, it so happensthat at 216 Megabits/second the information to be recorded by each oneof the four heads during 17 active horizontal picture periods nicelyfills 3 Random Access Memories (RAMs) each of 16k. Fortunately, 15blocks of 17 lines gives an appropriate number of lines (255) to recordon each field of the 525 lines standard; this would leave 71/2 linesduring field blanking unrecorded. It is also fortunate that 18 blocks of17 lines would leave only 61/2 lines recorded during field blanking onthe 625 line standard.

It is perhaps even more fortunate that both 15 and 16 are divisible by 3for this embodiment.

At the top of FIG. 2, which describes how the video bunching is arrangedfor 525 lines, is shown the input signal to the recorder. At the centreappears the signal as it is to be recorded on tape; equally this linecan also be considered to be the signal as recovered from the tape. Thelowest line shows the final output signal as it leaves the recorder. Theoverall delay between the top and the bottom line is one head wheelrotation, that is a third of a field. From video considerations it couldbe less but when audio is taken into account this appears to be theoptimum.

The arrows labelled V indicate the start of field blanking and justpreceed the lost 71/2 lines (61/2 on 625) shown shaded. The video datain each successive block of 17 lines is shuffled as it is stored in theRAM's and is later read out, at a time determined by the headwheelrotation, at 11/3 the input speed to bunch the data into a continuousblock of 5 by 17 lines which is recorded on tape. When a block of 17lines has been read out from a store the store may be re-used for laterblocks of 17 lines.

An analogue but reverse process is used in replay to stretch theinformation and delay it appropriately. The data is unshuffled by usingthe same shuffling technique for addressing the stores on replay read ashad been used for addressing the stores during the writing into thestores in the recording operation. The read and write operations fromthe stores at higher speed use ordinary sequential addressing.

FIG. 3 indicates an alternative way of recording the incoming digitalinformation. At the top of FIG. 3 is shown the input signal to therecorder and the lower line indicates the signal as it is to be recordedon the tape or as it is recovered from the tape.

As before, it is assumed that the tape is wrapped around the drum for270° but in this case only 3 heads are clustered together. The maindifference between this arrangement and the previous one is that thesize and number of the groups is altered. Previously, we had 3 groups of85 lines, in this embodiment we have 5 groups of 51 lines to make up onefield of 525 line system (6 groups would make up one field of a 625 linesystem). Irrespective of the standard, the groups would be identical;the only difference would be in the speed with which they are writteninto the buffer storage and read therefrom which would be in the ratioof 1000: 1001 for the 525 and 625 line standards respectively.

The size of the groups is open to variation. Obviously for this purposeone has to consider the number of digital words in a line as well as thenumber of heads and the desired bit rate per head. With the samplingfrequencies given above, there are 1440 8-bit words per line thus if 51lines are used there are a total of 73,440 words. We have determinedthat the minimum number of heads in contact with the tape at any onetime is two but that three or four heads is more likely to be the numberin practice. Further, it is possible to group the heads in pairs orthrees diametrically opposite to each other and use a tape wrap of 180°.If pairs of heads are used, there will be ten tracks recorded per fieldof a 525 line picture frame; this becomes 15 tracks if three heads pergroup are used. It would be possible to decrease the group size for thepaired head approach to 251/2 lines in which case there will be 20tracks per field of a 525 line frame. It is preferred that the number oftracks recorded be in the range of 10 to 20 tracks. This indicates thatthe size of a group be 17, 251/2, 51 or 85.

As mentioned before, an advantage of the system described above is thatthe audio signal associated with the video can be recorded in aconvenient manner. Referred to FIG. 2, it will be noted that there aredotted extensions at both ends of each group; this is where the audiowill be fitted in. On tape there will be an audio preamble followed bythe first audio digital data, a video preamble followed by 85 lines ofvideo data, broken up by framing information, and lastly a further audiopreamble followed by the second audio digital data. Individual preamblesare used for audio and video so that separate editing is possible. Forthe FIG. 3 arrangement, the video preamble will be followed by 51 linesof video data.

FIG. 4 is essentially of the same form as the FIG. 2 but considers theaudio rather than the video signals. The audio data that represents oneaudio signal during a time equal to one third of a field is bunchedtogether and is added on to the end of the video data, on one of theheads, with an intervening preamble; this bunching is illustrated by theleft hand shaded area between the top and centre lines, the one shadedby lines from bottom left to top right. The audio data from the samesource again during a time equal to a third of a field and substantiallyoverlapping that already considered is bunched together and preceeds thevideo data not on the same head but a different one. A different one isused so that the failure of a single head will not lose that audiochannel, it will only cause less satisfactory error concealment andediting. During each sweep one head allocation is used for the soundchannels for the first blocks of audio and a differing head allocationused for the second audio blocks.

On replay the bunched data is stretched out as shown by the similarlyshade areas and is delayed by the same amount as the video.

For a 270° video wrap the audio overlap is marginally longer than aquarter of a field so it is about 4.2 milliseconds on 525 lines. This isgreater than the minimum of 3.3 milliseconds considered necessary forthe duration of the cross-fade to achieve good click-free audio editing.This length of overlap occurs three times per field although forsimiplicity it is only shown once per field in this figure. For the FIG.3 arrangement, this length of overlap occurs five times per field.

A further advantage of the system is that error concealment isfacilitated. This is achieved by "shuffling" the data in the bufferstores prior to recording and conducting an opposite process on replay.

It is not intended to give a precise error concealment since any one ofa number of schemes could be used for example the one disclosed in ourU.S. Pat. No. 4,277,807. However, some general considerations will bediscussed in relation to the FIG. 3 embodiment.

Drop-outs can be so long that pure error correction schemes are notpracticable for the bit-rates under consideration. Error concealment hasto be used in this case; indeed it can be argued that since the numberof unconcealed errors using pure concealment is inherently lower thanthe number obtained using a mixed error correction/concealment strategy,it may be better to rely purely on error concealment. This subjecthowever is not germaine to the topic of formats except for oneconclusion which is that error concealment must be used either purely orin conjunction with error correction.

Since drop-outs may last hundreds and even thousands of words, it isnecessary that adjacent words of the input picture should be wellseparated on the tape so that words which may be used for concealment ofa word in error are unlikely to be affected by the same drop-out. Thisis an essential requirement but there is also considerable benefitobtained by spreading the effect of a drop-out so that a large area isaffected to a small extent rather than a small area being affected to amuch greater extent. The extent to which this "shuffling" of words betaken is debatable but it seems logical to extend it sufficiently farthat a longitudinal scratch on the tape will cause a uniformdistribution of concealment or correction over the complete picture.This will be achieved by shuffling the words over the whole of a 51 linegroup. Such shuffling requires a pair of buffer stores each with acapacity of 51 lines and since writing and reading cannot overlap andmust alternate in this case, there will be a delay of 51 lines.

Information during 51 lines will be written into the shuffling store andwill be read from it during approximately 39 lines of a standard 525line signal and recorded on tape. Using two shuffling stores there willbe a permissible float of 12 lines which arises from the difference intime for writing and reading. After allowing for retiming the signal tomake equispaced blocks for recording on tape there will remain some 4lines of float for other purposes, e.g., during synchronisingoperations.

During each line of the television signal a total of 1440 words, each ofeight bit, define the picture. Each of the three heads will thereforeneed to handle 480 words per line.

It would be convenient to organise the data into groups of 256 words.During picture in shuttle it will be necessary to identify which of the5 (6) groups in a field is being replayed by each head and also which ofthe three heads recorded that particular track. This gives a maximum of18 combinations for 625 lines. This will require 5 bits foridentification and it will be convenient to arrange information to occuronce in every group of 256 words; if one eight bit word be allocated forthis purpose there will be 255 words usable for recording video in eachgroup. There will also be three bits of this identifier word spare forother purposes.

It so happens that 255 and 480 are both divisible by 15 so each wordgroup will contain precisely 17/32 of the words needing to be recordedby one head in a line. During 17 lines there will be 32 words groups or96 word groups in a group of 51 lines for each head.

During a 15 line block for each head there will be 96×256 words which isequal to three times 8192. This is extremely fortunate since 8192 isprecisely 2 raised to the power 13 and a so-called 64k RAM organised asan 8-bit by 8k would just be completely filled by precisely one third ofthe words recorded by a head in a block of 51 lines This fortunatecoincidence makes the choice 51 lines per block and 3 heads in contactwith the tape very advantageous.

In order to obtain optimum performance in the presence of drop-outs orunder shuttle conditions each word group should contain words which areuniformly distributed over the 51 line/groups of the picture. If this bedone and if synchronising information precedes each word group itappears likely that a good picture in shuttle could be provided up toabout twenty five times normal speed but beyond this speed the qualityof the picture must inevitably start to degrade; this seems to beacceptable.

For 1440 words per line unless more than six heads are in contact withthe tape it is inevitable that regular distribution of the words betweenheads will result in vertically adjacent words being recorded by thesame head. This is undesirable since if a head becomes clogged, some ofthe words useful for concealing the missing words will also be missing.In this case the problem arises because 1440 is a multiple of 3, thenumber of heads. The problem can be overcome by causing an additionalstep in the normal regular distribution of words between head channels;this additional step must occur at least once per line and the range upto not more than twice per line is satisfactory. Since the word groupslast 17/32 of a line this additional step can be synchronised with thestart of each word group. Information regarding which of the threepossible phases starts a word group will also need to be included in theword group identifier and could use two of the three previously unusedbits in this word.

The stores used for shuffling in record and the deshuffling in replayare identical in principle and indeed the same store can be used forboth purposes. On the tape side of the store the read or the write cycletime of the RAMs is nominally 300 ns; during the other phase of storeoperation the cycle time would be 333 ns. This does not seem to beunreasonably fast.

These stores provide a time base correcting window for some four linesafter allowing for the systematic errors inherent in the basic system.This window is also available in record and could be used by way ofexample to allow for timing perturbations during synchronisingoperations or to correct for rapid angular accelerations of the body ofthe recorder during operation.

At present 64k RAMs are not available organised as 8 by 8k; 8 by 2k areavailable and it seems reasonable to expect that 8 by 8k will becomeavailable in the not too distant future.

All three heads for this track configuration will be mounted on a singleelectro-mechanical transducer which is an obvious advantage provided theadditional mass of the heads does not cause a significant reduction ofthe frequency of mechanical resonance.

The time available to fly back during slow motion in 833 us but this hasto be decreased by the amount of time necessary for synchronisation andthat required for audio. This may prove to be marginally too short but areduction from 270° to 260° would increase it to 926 us.

By advancing the confidence replay heads by six tracks during editingand by using separate shuffling stores for record and replay it would bepossible to realise fades and wipes rather than the cuts imposed byconventional analogue recorders if only one replay machine is available.

Advantages of the Proposed 270° 3-Head Configuration:

the same three heads are all used for every part of the picture, headbanding type of effects are therefore most unlikely.

the extent of shuffling is sufficient for a good picture in shuttle atall speeds up to a reasonable maximum; this shuffling extent alsoprovides optimum immunity to longitudinal tape scratches.

there is no sharing of any track between one field and the next; thiswill ease the problem if non-standard television signals are recorded.The philosophy of using 5 self-contained "groups" per 525 line field maymake the recorder more adaptable for non-television applications.

time-base correction window exceeding four lines on both record andplayback.

small number of heads and associated hardware.

However, there are two potential problems:

high lateral accelerations of heads during flyback of the trackfollowing servo in slow motion. This problem could be avoided by using afield store.

high rotational speed of head wheel, 300 revs/second. For completeness,FIG. 5 shows a block diagram for the record side of recording andreproducing apparatus utilizing the format shown in FIG. 3. Thereproducing block diagram is not given because it is felt that thedescription given in our U.S. Pat. No. 4,277,807 is sufficient for oneskilled in the art to modify it to reproduce the signal. It is to benoted however, that the error signal storage in this patent is notrequired for the present case.

Turning now to FIG. 5, incoming video data words to be recorded are inthe form of 8-bit words and are fed via write gating circuits 51, 52 toshuffling stores 53, 54 each of a capacity of fifty one lines. Thegating circuits 51, 52 are supplied with clock pulses at the input datarate and every 51 lines are of the gating circuits is opened in responseto a signal derived from the input line synchs. Preferably, the data iswritten into the stores in "shuffled" form, i.e., adjacent words are notloaded sequentially into adjacent memory locations and read out in asequential manner.

The reading out of data from the stores 53, 54 is controlled by readgating circuits 55, 56 each of which is supplied with clock pulses at arate higher than the input data rate whereby the stores are emptiedquicker than they are filled and preferably at a rate equivalent to 39lines a 525 lines standard. Additionally, each of the read gatingcircuits is controlled by a signal indicative of the headwheel positionand derived either directly from the headwheel or from its drivecircuitry (now shown).

The outputs of the shuffling stores are fed to parallel to seriesconverter circuits 58, 59 where the parallel data words are convertedinto a stream of serial bits and thence via a further gating circuit 60to the three recording heads.

I claim:
 1. Apparatus for recording or playing back a digital televisionsignal comprising a helical scanning device including a drum which maybe rotatable, a rotatable headwheel provided with a plurality ofrecording/playback heads, guide means for wrapping a magnetic recordingtape around a portion of the circumference of the drum, means forrotating the headwheel, and means for moving the tape around the surfaceof the drum whereby one field of a television video picture is recordedor played back by a plurality of rotations of the headwheel,characterised in that the recording/playback heads are disposed on theheadwheel and the degree of wrap of the tape around the drum is suchthat for one complete revolution of the headwheel there is a period oftime during which there is no head in operative contact with the tape,and storage means are provided for receiving information in the form ofdigital words to be recorded or played back for altering the timing ofthe information whereby on recording all the information received isrecorded and on playback the information on the tape is recovered andreturned to the timing relationship existing for the originalinformation.
 2. Apparatus according to claim 1, wherein all the headsare clustered together over one segment of the headwheel and the degreeof wrap is from 180° to 330°.
 3. Apparatus according to claim 2, whereinthere are at least three heads and the degree of wrap is between 240°and 300°.
 4. Apparatus according to claim 1, wherein there arediametrically opposed heads or head groups, and the rotation of theheads in which they are active for video is less than 180°.
 5. Apparatusaccording to claim 1, wherein storage means comprise a plurality ofstores for storing the digital words in the form of blocks of words,each block corresponding to a predetermined number of lines of a videosignal and the stores being written into at one speed and read from at asecond speed different to the first.
 6. Apparatus according to claim 5,wherein each store is capable of storing information relating to 51lines of a television video signal or a sub-multiple thereof. 7.Apparatus according to claim 5, wherein, for recording means areprovided responsive to timing signals in the digital information to berecorded for controlling the writing in of digital information into thedesired one of the stores, and means indicative of the position of theheadwheel are provided for controlling the reading of digitalinformation from a desired one of the stores.
 8. Apparatus according toclaim 1, wherein means are provided for supplying signals indicative ofthe audio information associated with the video to at least one of theheads for recording on the tape in the gaps between adjacent blocks ofvideo information.
 9. Apparatus according to claim 8, wherein said meansis arranged to time compress the audio information.
 10. Apparatus forrecording or playing back a digital television signal comprising ahelical scanning device including a drum which may be rotatable, arotatable headwheel provided with a plurality of recording/playbackheads, guide means for wrapping a magnetic recording tape around aportion of the circumference of the drum, means for rotating theheadwheel, and means for moving the tape around the surface of the drumwhereby one field of a television video picture is recorded or playedback by a plurality of rotations of the headwheel, characterised in thatthe recording/playback heads are disposed on the headwheel diametricallyopposite each other, storage means are provided for receivinginformation in the form of digital words representing video signals tobe recorded or played back for altering the timing of the informationwhereby, for one complete revolution of the headwheel there is a periodof time during which there is no head recording or playing back saiddigital words, thereby to record the video signals in blocks on the tapewith gaps therebetween, and means are provided for supplying signalsindicative of the audio information associated with the video signals toat least one of the heads for recording audio information on the tape ingaps between adjacent blocks of video information.
 11. Apparatusaccording to claim 10, wherein there are diametrically opposed headgroups, and the rotation of the heads in which they are active for videois less than 180°.
 12. Apparatus according to claim 10, wherein saidstorage means comprise a plurality of stores for storing the digitalwords in the form of blocks of words, each block corresponding to apredetermined number of lines of a video signal and the stores beingwritten into at one speed and read from at a second speed different tothe first.
 13. Apparatus according to claim 12, wherein, for recordingmeans are provided responsive to timing signals in the digitalinformation to be recorded for controlling the writing in of digitalinformation into the desired one of the stores, and means indicative ofthe position of the headwheel are provided for controlling the readingof digital information from a desired one of the stores.
 14. Apparatusaccording to claim 13, wherein said means is arranged to time compressthe audio information.